In recent years, the use of camera systems or code readers which are adapted to optically read labels containing alphanumeric or encoded information, such as bar codes or the like, has become widespread. The growth in the use of such camera systems or code readers has been particularly strong in the parcel delivery industry, where information is printed on optically-readable labels affixed to packages being shipped or transported. One example of a modern optically-readable label is described in U.S. Pat. No. 4,874,936, which is incorporated herein by reference.
In order to optically read labels in an automated fashion, the parcel delivery industry commonly uses "over the belt" camera systems. In such systems an overhead camera captures images of labels on packages traveling on a conveyor belt below the overhead camera. Typically these camera systems utilize charge-coupled device ("CCD") cameras. Modem over the belt camera systems are typically used with conveyor systems having belt widths of up to five feet to accommodate packages of varying sizes and shapes, and belt speeds of up to five hundred feet per minute.
Over the belt camera systems pose focusing challenges for their cameras. In order to capture and create images of labels containing alpha-numeric or encoded information, overhead camera systems must be able to focus on the labels as they rapidly pass on the conveyor belt. Packages traveling below an overhead camera often have varying heights, so the distance between the overhead camera and labels on the packages will change significantly. Often the height of the packages will vary to such an extent that some labels fall outside of the depth of field of the overhead camera, thereby preventing the overhead camera from adequately focusing on those labels.
Over the belt camera systems can also experience distortion problems. It is typical for an overhead camera to take "slices" of the image of a label as it passes below the camera. The width of each slice is determined by the field of view across the conveyor belt, and the length of each slice is determined by the velocity of the belt. The "aspect ratio" is defined as the ratio of the length of a slice to the width of a slice. It is important to keep the aspect ratio constant to prevent images from becoming distorted. In the parcel delivery industry, it is common for packages traveling on conveyor belts below overhead cameras to be of varying heights. Overhead cameras typically have a different field of view at different heights (i.e., at different optical path lengths). As a result, the "aspect ratio" varies between packages causing the images created by the overhead camera to become distorted.
It is common to avoid certain focusing and distortion problems in an over the belt camera system by adjusting the optical path defined between the overhead camera of the system and the packages passing under the camera. The optical path is adjusted so that the length of the optical path defined between the camera and each package is approximately the same (i.e., the length of the optical path is equalized). U.S. Pat. No. 5,185,822 discloses such an over the belt camera system, wherein a movable pair of mirrors 31, 32 move relative to and are interposed between an overhead camera 20 and a stationary mirror 6. The movable pair of mirrors face each other at an angle of 90 degrees and do not move relative to one another. The stationary mirror reflects an image of an object carried by the conveyor to a first mirror 31 of the movable pair of mirrors. The first mirror of the movable pair of mirrors reflects the image of the object to a second mirror 32 of the movable pair of mirrors. The second mirror of the movable pair of mirrors reflects the image of the object to the overhead camera.
While the over the belt camera system of U.S. Pat. No. 5,185,822 functions to equalize the length of the optical path defined between the camera and packages of varying heights, that system can also be described in the context of changing the length of the optical path defined between the camera 20 and a single, stationary object, such as a package. In that context, the movable pair of mirrors 31, 32 are together moved away from the stationary mirror 6 and the camera to increase the length of the optical path defined between the camera and the object. Also, the movable pair of mirrors are together moved toward the stationary mirror and the camera to decrease the length of the optical path defined between the camera and the object. The resulting changes in the optical path length are equal to twice the change in the distance between the stationary mirror and the movable pair of mirrors. Therefore, to substantially increase the optical path length, it is necessary to substantially increase the distance between the stationary mirror and the pair of movable mirrors and/or increase the number of mirrors interposed in the optical path. Increasing the number of mirrors increases the complexity of the system, and either option for increasing the optical path length consumes additional space.
U.S. Pat. No. 5,485,263 discloses another over the belt camera system, which includes an array of stationary mirrors 70 and a movable pair of mirrors 60, 80, for equalizing the optical path length defined between an overhead camera 20 and objects carried under the camera by a conveyor. The movable pair of mirrors move relative to one another and relative to the stationary array of mirrors. The movable pair of mirrors are each individually moved such that they together cooperate with a selected mirror of the stationary array of mirrors. Each mirror of the stationary array of mirrors is at a different distance from the movable pair of mirrors.
While the over the belt camera system of U.S. Pat. No. 5,485,263 functions to equalize the length of the optical path defined between the camera 20 and packages of varying heights, that system can also be described in the context of changing the optical path length between the camera and a single, stationary object, such as a package. Each mirror of the stationary array of mirrors 70 is at a different distance from the movable pair of mirrors 60, 80 and cooperates with the movable pair of mirrors to define a different optical path length. A first mirror 60 of the movable pair of mirrors directly receives an image from the object. That first mirror of the movable pair of mirrors reflects the image of the object to a first selected mirror of the stationary array of mirrors. The first selected mirror of the stationary array of mirrors reflects the image of the object to a second mirror 80 of the movable pair of mirrors. The second mirror of the movable pair of mirrors reflects the image to the camera. The optical path length is changed by selecting and having the movable pair of mirrors cooperate with a second mirror the stationary array of mirrors. The change in the optical path length is equal to twice the distance between the first and second selected mirrors of the array of stationary mirrors. To substantially increase the optical path length, it is necessary to substantially increase the distance between mirrors and/or increase the number of mirrors interposed in the optical path. Having additional mirrors interposed in the optical path increases the complexity of the system, and either option for increasing the optical path length consumes additional space.
In summary, prior optical path equalizers require numerous mirrors in the optical path and/or substantial distances between mirrors to compensate for substantial variations in package heights. Having numerous mirrors interposed in the optical path detrimentally increases the complexity and size of optical path equalizers. Having substantial distances between mirrors of optical path equalizers also detrimentally increases the size of optical path equalizers.
There is therefore a need for an optical path equalizer that can compensate for substantial variations in package heights while occupying a small space and not requiring an excessive number of mirrors.